CA2211838C - Dispersion compensation in optical fibre transmission - Google Patents

Abstract

An optical fibre transmission system comprises an optical signal source operable to generate an optical signal at a predetermined bit rate and at a signal wavelength; an optical fibre transmission link connecte d at a first end to the signal source, the link having dispersion characteristics at the signal wavelength; an optical amplifier serially disposed in the link; and a signal receiver connected at the second end of the link; in which a grating is connected in the link, the grating being chirped by an amount providing at least partial compensation of the dispersion characteristics of the link, the compensation being such as t o provide a signal, at the second end of the link, compatible with the sensitivity requirements of the receiver at the second end of the link.< /SDOAB>

Description

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DISPERSION COMPENSATION IN OPTICAL FIBRE TRANSMISSION
This invention relates to dispersion compensation in optical fibre transmission.
Data transmission in optical fibres is generally limited by power loss and pulse dispersion. The advent of erbium-doped fibre amplifiers (EDFAs) has effectively removed the loss limitation for systems operating in the third optical communication window (around a wavelength of about 1.55~cm (micrometer)), leaving pulse dispersion as a serious limitation, especially in future high-capacity mufti-wavelength optical networks.
More importantly, most fibre which has already been installed for telecommunication links (ie. standard non- dispersion shifted fibre) exhibits a dispersion zero around 1.3~.m and thus exhibits high (about l7ps/nm.km (picosecond per nanometre-kilometre)) dispersion around 1.55~,m. Upgrading this fibre to higher bit rates involves the use of EDFAs and a shift in operating wavelength to 1.SS~.m where dispersion-compensation becomes a necessity.
Several techniques have been demonstrated including laser pre-chirping (reference 1 - below), mid-span spectral-inversion (phase-conjugation) (reference 2 -below), the addition of highly-dispersive compensating fibre (reference 3 -below) and chirped fibre gratings (references 4 to 7 - below). Chirped fibre gratings are of particular interest, since they are compact, low-loss and offer high negative-dispersion . of arbitrary and tunable profile. In separate experiments 450fs (femtosecond) pulses have been successfully reconstructed after transmission through 245m of fibre (reference 4 - below), and gratings with dispersion equivalent to 20km and lkm of standard fibre have been fabricated (references 5 and 6 - below). Whilst more recently a grating has been employed to compensate the dispersion of 160km of standard fibre in a lOGbitsu (gigabits per second) externally modulated experiment (reference 7 - below) although no information of the grating strength was given in this case.
It is a constant aim to improve dispersion compensation techniques in optical , fibre transmission systems.
The article in IEEE Photonics Technology Letters, April 1993, USA, vol. 5, no. 4, pages 425-427, Farre J et al: "Design of bidirectional communication systems AMENDED SHEET

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with optical amplifiers", discloses the use of optical amplifiers at various positions in an optical fibre link.
The article in Optics Letters, vol. 19, no. 23, 1 December 1994, Washington US, pages 2027-2029, Lauzon et al: "Implementation and characterization of fiber Bragg gratings linearly chirped by a temperature gradient", discloses (as the title suggests) the manufacture of chirped fibre gratings by imposing a temperature gradient onto a fibre grating.
GB-A-2 161 612 discloses a chirped fibre grating for dispension compensation in an optical fibre link.
This invention provides an optical transmitter for use with an optical fibre transmission link, the transmitter comprising:
a Iight source capable of direct or indirect modulation; and an optical amplifier;
characterised by:
a chirped grating to provide compensation for the dispersion characteristics of the link over the range of wavelengths of the modulated light source.
Preferably the optical amplifier is operable in a saturation mode.
It is advantageous to position the compensating grating at the input end of the link, since in this position the optical input signal is still relatively large and thus a relatively insignificant noise penalty is incurred. In addition, if the grating's (compensated) output is then routed to an optical amplifier operating in saturation, the amplifier's output power will be effectively unaltered by the presence of the compensating grating.
The skilled man will appreciate that the dispersion compensation in this context need not be complete, but simply that the non-linear response of the grating acts against the dispersion characteristics of the transmission link.
This invention also provides an optical fibre transmission system comprising:
an optical fibre transmission link; and an optical amplifier disposed at an input end of the link;
characterised by:
a chirped grating disposed at the input end of the link, the chirped grating providing compensation against the dispersion characteristics of the link.
AME(~DED S~iEET

The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which:
Figure 1 is a schematic diagram of a dispersion compensating optical fibre transmission system;
6 Figure 2 schematically illustrates the spectrum of a DFB (distributed feedback) laser transmitter;
Figures 3a to 3c schematically illustrate the reflectivity spectra of a fibre grating as written (Figure 3a), with a temperature gradient set to add to the existing chirp (Figure 3b) and with a temperature gradient set to reverse the existing chirp (Figure 3c);
12 Figures 4a to 4c schematically illustrate the time delay of the gratings of Figures 3a to 3c respectively;
Figures Sa and Sb schematically illustrate sampling oscilloscope traces of an approximately lops, 0.318nm spectral halfwidth signal after propagation through SOkm of standard fibre without compensation (Figure Sa) and with compensation (Figure Sb);

Figure 6 schematically illustrates bit error rate (BER) curves for the system of Figure l;

Figure 7 schematically illustrates a transmission penalty at a 10-9 BER as a function of span length with and without dispersion compensation; and G
Figures tea to ~f schcmatic<ally illustrate eve diagrams showing the different results obtained without (Figures ~a to Hc) and with (Figures Hd to Sf) dispersion compensation.
Referring now to Figure 1, in this cmhc»iimcnt a chirped fihrc grating 1f) was incorporated into a ?.SGbits-1 directly-meolulatcd system operating at 153Cnm.
, (However, in other cmhodimcnts and in the ~lc~cription hclow, an indirectly modulated transmitter could be used instead). As a consequence of the direct modulation the output of the DFB laser transmitter ?t> was chirped and cxhihitcd a 3dB
optical bandwidth of ().lnm and a 1t)dB (dccihcl) lrlmlwidth of ().?-lnm, is cduivalcnt to a 1() lUGbit/s modulation signal.
The transmitter 3() was aupplic~l with data from a commercial multiplexer (not Sh()WIl~ from a Phillips SDH (synchronous ~li~~ital hierarchy) ?.SGhits-1 system. The multiplcxcr combines 1h channels of data at 1-It)Mhits-1 (megahits per second) up to a line rate of ?.SGhits-t. In the ahscnce of ~Uta on any channel, the multiplexer generates random data. Random data was input to several of the channels whilst, on the test channel, pscudorandom data at l:~()Mhits-t with a ?'~-1 pattern length (generated by a BER test set 11 ()) was cmplovc~f. However, in a real application, it will of course he appreciated that real input data would he supplied to the transmitter instead of the pseudorandom data from the BER test set.
?U The transmitter ?t) c()l7slstC~l of a directly-modulated DFB laser with wavelength centred at 153Onm and whose chirped output had a 3dB bandwidth of ().1()8nm and 1()dB bandwidth of ().?.lnm. The spectral characteristics of the transmitter output are illustrated schematic:_Illy in Figure ?. As a consequence of this chirp (and the fibre dispersion) a penalty was of~scrvcd for transmission distances in standard fibre in excess of a few tens of km.
The transmitter was followed by a single-stage, ~)~()nm-pumped erbium-doped power-amplifier 30 giving an output power of +l2dBm (decibels relative to 1 milliwatt) which was transmitted through standard fibre having lengths of 100, and 2U()km. In the latter case (as illustrated in Fiy~urc 1), a dual-stage 980nm-3() pumped line amplifier 4() giving an output power of +l3dBm was incorporated between two series connected 1()()km len~~ths of fihrc 5(), ().
The output of the link was coupled via a variahle attcnuator 70 to a commercial, Phillips, receiver and dcmultiplcxer t~(), the output of which was in turn passed to the BER test set 11() for BER measurement (by comparison with the test data supplied to the transmitter ?() by the BER ;.~cncrator 11()).
Dispersion-compensation of the link was provided by incorporating the chirped fibre grating 10 between the trap smittcr ?() and power amplifier 3(). Since the grating 1(> operates in reflection, an optical circulator ~)() was included to convert it to a transmission device. The grating was connected to the circulator using so-called NTT
FC/PC compatible connectors (not shown). However, to ensure successful operation, index matching liduid (not shown) Wa1 111SC1'tCCI in the connection to minimise reflections.
Power levels in the link arc such that it is operWiny~ in the so-called linear regime, thus the dispersion compcns,ction could in theory he performed at any location in the link. However it is ac.ivanta~,eous to incorporate the grating in its present location (before the fibre lengths >(), W)) since the input si~~nal to the power amplifier is still relatively large and thus a relatively insignificant noise penalty is incurred. In addition, since this amplifier 3() is then operating in saturation the output power will be effectively unaltered. Alternatively, if the dispersion compensation had been included immediately prior to the receiver .1 pcnnlty would have been incurred due to its insertion loss.
~() The fibre grating was written usin~~ standard tcchniducs with a frequency-doubled excimer laser in a gcrmania-boron co-doped fibre (().1 NA (numerical aperture), lam ~.~~~~rr (cutoff wavelength)). The grating was shout 20mm in length with an approximately Gaussian strength profile and about 7()~/o peak reflectivity. In its "as-written" state it had sonic residual chirp and a measured bandwidth of about O.Znm. The grating was further chirped to a 3d8 hane.lwidth of about ().3nm by applying a linear temperature gradient. The temperature ~,rac(ient could he applied to either add to or reverse the existing chirp.
Surprisingly superior performance was obtained when the temperature gradient was applied to reverse the existing chirp of the y~rating. This was due to the slightly non-linear characteristics of the existing chirp.
In other words, the chirped optical fibre grating is formed by applying a temperature gradient to a portion of optical fibre on which a non-linear grating is ij impressed, the variation induced h)~ the temperature gradient actin' against the non linear variation of the impressed grating, and in particular where the temperature gradient at Icast negates the non-linear variaticm of the impressed grating, thereby generating a grating having a non-linear variation in the opposite sense to the impressed grating.
Figures 3a to 3c show the grating spectral response as written (Figure 3a), with the temperature gradient set to add to (Fi~~urc 3h) and reverse (Figure 3c) the existing chirp. A slight dip is noted in Figure 3h.
Figures 4a to =Ic show the time dclav of the ~~ratings, measured using an standard intcrferomctric set up, corresponding to the respective cases illustrated in Figures 3a to 3c. (Since the measurements were pcrformecl cm different instruments there is a slight mismatch in indic,ncd w~,mclcn~~ths. Also, all three measurements were taken from the same end of the aerating and thus in the aerating of Figure 4b the grating was tested by the interferometer in the opposite direction to its direction of use in the embodiment of Figure I).
As stated, case h (i.c. as sho~m in Figures 3h and ~h) did not tend to give stable link performance and thus a temperature profile I ()(> indicated in Figure 1 (case c, i.e. as indicated in Figures 3c and =Ic) was employed. The centre wavelength of the chirped grating was also tuned to match the Ic.lscr wavclcn_~Tth of 1536nm.
Once 2() chirped, the grating reflectivity reduced and thus the circulator-grating combination exhibited an insertion loss of 3.5dB, hut owin~~ to its location at the output of the transmitter 20 and before the amplifier 3(), this had a negligible effect on the link's power-budget.
Separate measurements involving compensation for the propagation of about lops pulses over 5() and 1()(>km using this ~~rc~ting showed structure in the compressed pulses, indicating phase-distortion of the pulses and thus non-perfect compensation of the dispersion. However, owing to the non-transform-limitcc.l data (chirped source) an Improvement in system performance was nevertheless obtained.
Figure 5a shows a sampling oscilloscope trace of an approximately lops, U.31$nm spectral halfwidth pulse after propagation through 5()km of standard fibre.
The pulse can be seen to have broadened to about 2hlps. After rccompression with the grating, Figure 5b, the pulse width 11 SCCIl to he reduced to about 39ps.
However, V1'O 96/23372 PCT/GB96/00189 structure can be seen, particularly on the lc~ulin~~ cc.igc of the pulse which might be detrimental at higher hit rates.
Bit-error-rate (BER) curves for the system arc shown in Figure h. Data are ' given for back-to-back anti direct transmission through 1()(), 113 and ?t)()km of standard fibre. Dispersion-cyualiscd curves, with the chirped grating included, are d given for back-to-back and transmission thruu~~h 1t)() and ?()()km of fibre.
In the cast of direct transmissi«n, a trick-t«-hack sensitivity of -3''.7dBm at a It)-'' BER is observed. At this error rate a penalty of l.3dB was found at 1t)Okm, increasing to 3.? and R.SdB at 1=I3 and 3()()km, respective(.
1() The increase in penalty with distance is shown again in Figure 7. With the grating incorporated, the hack-to-hack sensitivity is actually improved by l.2dB, since the grating compresses the chirped-source pulses. The ~~ratin~~ virtually eradicates the penalty at 1()()km ((>.SdB) and significantly reduces the penalty at ?()()km to only 3dB.
No floor in the error-rate curves was observed when using the grating.
The increase in penalty with distance in this case can he compared with the direct result in Figure 7, where it can he seen thc.vt the gratin' dispersion is cduivalent (but opposite in sign) to around lU)km of stand,rrd fibre. This result is in substantial agreement with the ciclay data illustrated in Figure Vie.
Receiver eye diagrams arc shogun for various points in the system in Figure 8.
Although the interpretation of cyc diagrams is always subjective, the skilled man will appreciate that the beneficial effect of using the gratin, 1 () can he seen.
In summary, dispersion-compensation using a chirped fibre grating has been successfully demonstrated in a 2()()km standard-fibre transmission experiment using a 2.SGbits-1 1.55,um directly-modulated transmitter. The about 2()mm long, ().3nm chirped grating 10 effectively compensated for about h()km of standard fibre (i.e. fibre having a dispersion zero around 1.3~cm and about l7ps/nm.km dispersion around 1.55~cm), as anticipated. Thcsc results demonstrate that a non-uniformly chirped grating could provide significant improvements in current, directly modulated commercial systems.
Thus, an approximately 2()mm (millimetre) long grating with substantially linear chirp, to give a ().3nm 3dB bandwidth, substantially negates the dispersion of about 60km of standard fibre. This allowed transmission through 2()()km of standard fihre with a 3dB penalty, which compares with an approximntcly ~.~cIB penalty without the compensation.
In summary, therefore, cmhodimcnts of the invcntic>n make use of a grating is connected (in a dispersion compensating fashicm c.g. using an optical circulator) in an S optical fibre link, where the grating is chirpec.l by an amount providing at least partial compensation of the dispersion characteristics of the link, to provide an output signal from the link compatihlc with the sensitivity rcduiremcnts of the receiver at the second end of the link. In particular, in emf~odimcnts c>f the invention, the optical signal at the output end of the link ca.cn he made to have a dispersion causing a penalty in 1I) sensitivity at the receiver of Icss than h.> dccihcls at a hit error rate of 1 ()-'~ in a link longer than 2()() kilometres.

Publication References 1. B. Wedding, B. Franz and B. Jungincr, ' "Dispersion supported transmission at 1()Gbit/s via up to '?53km of standard single-mode fibre", Proc. ECOC, Scpt 1?-1h, 1~)~)3, pupa TuC-1.3.
?. R.I. Laming, D.J. Richardson, D. Tavcrncr and D.N. Paync, "Transmission of (cps linear pulses over st)km of standard fibre using mid-1() point spectral inversion to eliminate dispersion."
IEEE Jnl. of Quantum Elcctrcmics, Vol, 3t), 1 ~)~)~., pp.'' 1 14-? 11 ~).
3. M. Onishi, H. Ishikawa, T. Kaahiw~.id;i, K. iVakazato, A. Fukuda, H.
Kanainori anti M. Nishimura, "High performance dispersion-compensating fiber and its application to upgrading of 1.31,um optimized system", Proc. ECOC, Scpt 13-1(,, 1~)~)3, paper WcCfi.S.

4. R. Kashyap, S.V. Chernikov, P.F. McKcc and J.R. T..~ylor, 2() "30ps chromatic dispersion compensation of 4t)()fs pulses at 1()OGbits/s in optical fibres using an all fibre photoin~lucc~l chirpc~l reflection grating", Electronics Letters, Vol. 3t), No. 13, pp. 1()71;-1()Ht), l~)~)~l.

5. KØ Hill, F. Bilodcau, B. Malo, T. Kitagawa, S. Thcriault, D.C. Johnson, J.
Albert and K. Takiguchi, "A periodic in-fibre Bragg grating's for optical fibre dispersion compensation", Proc. OFC'94, PD2, pp.l7-2().
G. J.A.R. Williams, I. Bennion, K. Su~~dcn and N.J. Doran, "Fibre dispersion compensation using a chirped in-fibre gating", Electr. Lett., Vo1.30(12), 1994, pp.~)1~5-9H7.

7. D. Garthc, W.S. L,cc, R.E. Epworth, T. Bnchcno and C P Chcw, "Practical dispersion cyualiscr based on fibre gratings with a hit-rate length product of 1-li TB/s.km"
Proc. ECOC Vol. ~ (Postdcadline papers), pp.l l-1~, Sept.?5-29, 199, ' ia. B. Malo, K.O. Hill, S. Thcriault, F. Bilodcau, T. hita~~awa, D.C. Johnson, J.
Alhcrt, K. Takiguchi, T. Kut~u~ka and K. Ha~~imoto, "Dispersion compensation of n 1()()kn~, I()Ghit/s optical fiber link using a chirped in-fiber Bragg ~;ruting with a lincnr dispcraicm characteristic", 1(.) ihid., pp. ?3-3h.

Claims (13)

CLAIMS:

1. An optical transmitter for use with an optical fibre transmission link, the transmitter comprising:
a light source capable of direct or indirect modulation; and an optical amplifier;
characterized by:
a chirped grating to provide compensation for the dispersion characteristics of the link over the range of wavelengths of the modulated light source.

2. A transmitter according to claim 1, in which the amplifier is disposed between the grating and the transmission link.

3. A transmitter according to claim 2, in which the amplifier is operable in a saturation mode.

4. A transmitter according to any one of claims 1 to 3, in which the grating is a fibre grating.

5. A transmitter according to claim 4, in which the grating is a reflection fibre grating.

6. A transmitter according to claim 4, in which the chirped grating is formed by applying a temperature gradient to a portion of optical fibre on which a non-linear grating is impressed, the variation inducted by the temperature gradient acting against the non-linear variation of the impressed grating.

7. A transmitter according to claim 6, in which the temperature gradient at least negates the non-linear variation of the impressed grating, thereby generating a grating having a non-linear variation in the opposite sense to the impressed grating.

8. A transmitter according to any one of claims 1-3, comprising an optical circulator for receiving the modulated optical output of the light source, the circulator being connected to route the modulated optical output to the chirped grating and to route optical signals from the chirped grating to an output port;
the optical amplifier being connected to receive optical signals from the output port of the optical circulator.

9. An optical fibre transmission system comprising:
an optical transmitter according to any one of claims 1-3, the transmitter being operable to generate optical signals in dependence on input data;
an optical fibre transmission line for propagating optical signals generated by the transmitter; and an optical receiver for converting optical signals output from the transmission link into corresponding electrical data signals.

10. An optical fibre transmission system comprising:
an optical fibre transmission link; and an optical fibre amplifier disposed at an input end of the link;
characterized by:
a chirped grating disposed at the input end of the link, the chirped grating providing compensation against the dispersion characteristics of the link.

11. A system according to claim 10, in which the optical signal at a distal end of the link has a dispersion causing a penalty in sensitivity at the receiver of less than 8.5 decibels at a bit error rate of 10 -9 in a link longer than 200km.

12. A system according to claim 10, in which the optical amplifier is operable in a saturation mode.

13. A system according to claim 10, comprising a variable attenuator connected between the output of the transmission link and the optical receiver.